Aircraft turbine engine main fuel pumps are typically high-pressure positive-displacement pumps in which the pump flow rate is proportional to engine speed. At many engine operating conditions the engine flow demand is significantly less than the high amount of flow supplied by the main fuel pump. The excess high-pressure pump flow is typically bypassed back to the low pressure inlet. Raising the pressure of the excess flow and then bypassing it back to the low-pressure typically wastes energy. Generally, this wasted energy is converted to heat, which flows into the fuel and results in undesirably high fuel temperatures.
Systems that use two fuel supplies, for example two positive displacement pumps, can be used to minimize the amount of bypass flow at high pressure differentials. This can be done by separating the two supply flows and only bypassing flow from one pump at a high pressure differential (e.g., the second supply pump would be bypassed at a much lower pressure differential). This reduces the wasted energy (i.e., heat) added to the fuel.
One problem encountered in implementing fuel distribution systems with dual pump supplies is that when the second pump supply is added (or subtracted) to the first pump supply, the system often generates unacceptable flow transients resulting from the switch between single-supply and dual-supply operating modes.
It would therefore be desirable to have a system and method for dual-supply fuel distribution that reduces the flow transients which normally occur during transitions between single-supply and dual-supply operating modes. Embodiments of the invention provide such a system and method. These and other advantages of the invention, as well as additional inventive features, will be apparent from the description of the invention provided herein.